1. Field of the Invention
The present invention relates to an endoscope and, more particularly, to an endoscope which has as an image pick-up device a plate-like image sensor disposed within a viewing head thereof and is capable of outputting an optical image obtained by an objective lens assembly in the form of a video electric signal.
2. Description of the Prior Art
In addition to endoscope of a type having an optical fiber bundle as its image guide means, conventionally there have been proposed TV endoscopes of a type employing as an image pick-up device a plate-like image sensor formed by a combination of charge transfer elements, such as a charge coupled device (CCD) comprising a great number of small photosensitive picture elements (pixels) arranged in matrix. Such TV endoscopes are found advantageous over the above-mentioned conventional endoscopes employing the optical fiber bundle as the image guide means thereof in that they have a longer durability, they are capable of processing an electric signal as a video image output in various ways, and they can be produced at lower costs. Therefore, the practical application of the TV endoscopes is now under consideration.
Now, the TV endoscope is requested to satisfy two requirements; one requirement is to reduce the outside diameter of the endoscope to the degree that it can be inserted into a cavity of a living body; and, the other is to improve the resolving power of the image sensor, that is, to increase the number of picture elements on the image sensor. Thus, it becomes a very important point how to construct the forward end portion of the endoscope, especially how the image sensor of an objective lens assembly permitting the image sensor to form an optical image is arranged efficiently in space.
Accordingly, in order to arrange a plate-like image sensor without enlarging the forward end portion of the endoscope, the present applicant has already proposed an arrangement in which the plate-like image sensor is provided in a plane including the center axis of the endoscope in the longitudinal direction thereof, an objective lens assembly is disposed on one side of the plate-like image sensor with the axial line thereof being parallel to the axial line of the plate like image sensor, a right-angled prism is provided in the rear of the objective lens assembly to turn a light path at 90.degree., and a light emitting surface is cemented to the image sensor (Japanese Patent Publication No. 104915 of 1985). This arrangement permits the effective use of the outer diameter of the endoscope, making it possible to provide a wide image sensor in the endoscope forward end portion of limited diameter.
By the way, to observe an affected part through an objective lens assembly, it is necessary to irradiate the affected part by use of light guides. Therefore, the light guides each formed of an optical fiber bundle or the like are provided on opposite sides of the objective lens assembly in parallel thereto in the forward end portion of the endoscope. In other words, the light guides are disposed close to the light receiving surface of the image sensor. The optical fiber bundle forming the light guide is covered with a tube of silicone rubber or the like and is inserted into the endoscope. However, since the forward end portion of the optical fiber bundle must be fitted into the opening of a metal barrel, the tube is peeled off from the forward end portion of the optical fiber bundle or the light guide, providing an exposed portion in the light guide which is not covered with the tube. Thus, in the prior art, such exposed portion of the light guide is hardened by means of epoxy resin or the like to prevent it from getting loose. As a result of this, there is a possibility that the light of the light guide may leak from the exposed portion to the light receiving surface of the image sensor. If any light other than the light from the objective lens assembly enters an image area forming the image pick-up area of the image sensor, then the image pick-up function of the image sensor is impeded. Also, if any light other than the light contributing to the image formation enters a storage area serving as an image transfer area, then the transfer function of the image sensor is impaired. Consequently, inaccurate images may be obtained or images obtained may be blurred due to blooming or the like.
To eliminate the above-described disadvantages, the applicant has proposed an endoscope in Japanese Utility Model Application No. 130217 of 1984 in which a light shielding plate or a mask is interposed between an image sensor and an objective lens assembly and is formed with a round hole in the portion thereof where a light from a prism in the objective lens assembly enters, whereby lights are prevented from entering from light guides and thereby only the optical images that are obtained from the objective lens assembly can be received by the image sensor.
It is true that the endoscope disclosed in the above-mentioned application is found satisfactory in eliminating the drawbacks found in the before-described prior art endoscopes. But, in the above endoscope, when the light shielding plate is cemented to the image sensor, if the center of the round hole of the light shielding plate is set out of alignment with the center of the image area of the image sensor, then an observed image is projected on a monitor television off from the center thereof. For this reason, the light shield plate must be accurately aligned with the image area. Also, if the center of the round hole of the light shielding plate is set out of the center of the prism in the objective lens assembly, the optical image from the objective lens assembly is then shielded in part by the light shield plate and thus the observed image is projected on the monitor TV in a manner that it is partially cut away. Therefore, it is not easy to assemble the endoscope including the accurate alignment of the objective lens assembly, light shield plate and image sensor. Further, the objective lens assembly and the image sensor are cemented to each other by means of a transparent adhesive resin. However, the transparent adhesive resin may produce air bubbles which give rise to the diffused reflection of a light incident from the prism.